## isokinetic relationship

https://doi.org/10.1351/goldbook.I03285
When a series of structurally related substrates undergo the same general reaction or when the reaction conditions for a single substrate are changed in a systematic way, the @[email protected] and @[email protected] sometimes satisfy the relation: $\Delta ^{\ddagger}H - \beta \ \Delta ^{\ddagger }S = \text{constant}$ where the parameter $$\beta$$ is independent of temperature. This equation (or some equivalent form) is said to represent an 'isokinetic relationship'. The temperature $$T=\beta$$ (at which all members of a series obeying the isokinetic relationship react at the same rate) is termed the 'isokinetic temperature'. Supposed isokinetic relationships as established by direct correlation of $$\Delta ^{\ddagger}H$$ with $$\Delta ^{\ddagger}S$$ are often spurious and the calculated value of $$\beta$$ is meaningless, because errors in $$\Delta ^{\ddagger}H$$ lead to compensating errors in $$\Delta ^{\ddagger}S$$. Satisfactory methods of establishing such relationships have been devised.
See also:
compensation effect
,
isoequilibrium relationship
,
isoselective relationship
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1129 [Terms] [Paper]